1. Field of Invention
This invention relates to a method and system for compensating for residual dispersion curvature.
2. Description of Related Art
Dispersion is a known phenomenon in optical communication networks that causes a broadening of input pulses traveling along the length of the fiber. One type of dispersion relevant to the invention is chromatic dispersion (also referred to as “material dispersion” or “intramodal dispersion”), caused by a differential delay of various wavelengths of light in a waveguide material.
Dispersion has a limiting effect on the ability to transmit high data rates. When modulated onto an optical carrier, an optical spectrum is broadened in linear proportion to the bit rate. The interaction of the broadened optical spectrum with wavelength-dependent group velocity (i.e., dispersion) in the fiber introduces signal distortions. The amount of tolerable distortion is inversely proportional to the square of the bit rate. Thus, the combination of increasing spectral broadening and decreasing distortion tolerance makes the overall propagation penalty proportional to the square of bit rate.
This results, for example, in a 10 Gbps signal being 16 times less tolerant to dispersion than 2.5 Gbps signal, while having only 4 times the bit rate. Dispersion accumulates linearly with propagation distance in the fiber and typical propagation distances in standard single-mode fiber (e.g., SMF-28 or equivalent) are ˜1000 km at 2.5 Gbps, 60 km at 10 Gbps, and only ˜4 km at 40 Gbps. Clearly, some form of dispersion compensation is required to obtain meaningful propagation distances at bit rates of 10 Gbps and above.
Fiber-optic system transport capacity has been increasing through combining multiple, separately modulated optical carriers at distinct wavelengths onto a single fiber. This technique is known as wavelength-division multiplexing (WDM). Due to WDM, it is preferable that dispersion compensation be performed for multiple wavelengths using a common device.
Several methods have been proposed to compensate for dispersion, including fiber Bragg gratings, optical all-pass interference filters and dispersion compensating fiber. Dispersion compensating fiber (DCF) has found widespread practical acceptance and deployment due to numerous advantages. Such advantages include relatively low loss and cost and the ability to simultaneously compensate channels across multiple wavelengths without requiring spatial separation. Further, DCF has the ability to compensate for the unavoidable variation in the dispersion as a function of wavelength (second-order dispersion or dispersion slope) that exists in many current transport fibers.
To compensate for positive dispersion in a transmission fiber, conventional systems use lengths of DCF that have a negative dispersion coefficient. The length of DCF is selected so that the negative dispersion produced by the DCF counteracts the positive dispersion in the transmission fiber. While DCF provides adequate levels of dispersion compensation, it is difficult to produce DCF that also simultaneously compensates the dispersion slope. As transmission lengths between regeneration points increase and data rates increase, the need to compensate dispersion slope is paramount. Uncompensated dispersion slope results in system penalty and can significantly shorten transmission distances and/or channel counts. Ideally, upon reception each channel should have the same amount of net dispersion so that the net dispersion slope is zero.
Some types of DCF provide a high level of dispersion slope compensation. Unfortunately, this high level of dispersion slope compensation can introduce residual dispersion curvature. FIG. 1 is a graph of dispersion versus wavelength for a transmission fiber (referenced as E-LEAF), a first type of DCF (referenced as Type II DCF) and a second type of DCF (referenced as Type III DCF). The dispersion profile for the second type of DCF has a steeper slope and thus can provide a higher degree of dispersion slope compensation. Such transmission fibers and dispersion compensating fibers are manufactured by suppliers such as Corning, Lucent, and OFS-Fitel. In particular, the E-LEAF fiber described above is a popular transmission fiber manufactured by Corning.
FIG. 2 is a graph of residual dispersion versus wavelength for the two types of DCF referenced in FIG. 1. The graph shows the remaining dispersion levels after each type of DCF is used to compensate the 75 km of transmission fiber. As is evident, the residual dispersion after using the first type of DCF (referenced as Type II DCF) is linear, but the level of dispersion is significant, particularly in higher wavelengths. This high residual dispersion is problematic for high wavelengths in the C-band (1520 nm-1568 nm) and L-band (1568 nm-1610 nm).
The residual dispersion after using the second type of DCF (referred to as Type III DCF) has a lower magnitude across the wavelength range, but exhibits quadratic profile with respect to wavelength. Although the effect of residual dispersion and its curvature may not seem large in FIG. 2, the effect is additive when spans are concatenated. FIG. 3 is a graph of residual dispersion versus wavelength for transmissions over multiple, concatenated spans.
As evident from FIG. 3, as the number of spans increases, the quadratic nature of the residual dispersion increases. This gives rise to “dispersion shaping” or the reduction in effective transmission bandwidth since channels located at both the short and long wavelengths experience substantial amounts of uncompensated dispersion, relative to channels in the center. Although the channels near 1550 nm are well compensated, the curve can be linearly translated such that the zero dispersion point may be made to correspond with either the short or long wavelengths. The shape of the residual dispersion curve, however, does not change thereby reducing the effective bandwidth.